This application will investigate whether: 1) The K+ channel underlying the resting membrane potential of DA smooth muscle changes during development from a calcium-dependent K+ channel to a voltage-gated delayed rectifier channel; 2) Increased oxygen inhibits the voltage regulated K+ channels, but not calcium-regulated K+ channels, to initiate vasoconstriction; and 3) The change in the K+ channel setting the resting membrane potential determines the stage in fetal development at which the DA constricts to oxygen. Potassium channels will be studied in fetal rabbit DA cells using both whole-cell and single channel patch-clamp techniques. Functional studies will be carried out on isolated DA vascular rings.